The present invention relates to a method of making cemented carbide powders with low compaction pressure, in particular submicron- and nano-sized powders.
Cemented carbide is made by wet milling of powders forming hard constituents, powders forming binder phase and pressing agents (generally PEG or polyethylene glycol) to a slurry, drying the slurry generally by spray drying, tool pressing the dried powder to bodies of desired shape and finally sintering. During sintering the bodies shrink about 16-20% linearly. The shrinkage depends on the % of theoretical density achieved during compaction of the powder to produce the green body (=“green density”), which in turn depends upon pressing pressure, WC grain size, grain size distribution, Co-content, and pressing agent. Pressing tools are expensive to make and are therefore made for a standard shrinkage such as 18%. The shrinkage is obtained by applying sufficient pressing pressure to the compact so as to give the desired green density. It is extremely important that the sintered body has a size as close as possible to that desired in order to avoid expensive post sintering operations such as grinding. However, if the grain size is fine, for example one micron or less, a higher pressing pressure is needed to obtain the necessary shrinkage. It is thought in the industry that increasing internal friction within carbide powders of decreasing grain size causes greater resistance to compaction. A high pressing pressure is not desirable because of a greater risk of pressing defects such as cracks or pores in the pressed bodies, abnormal wear of the press tools and even risk of pressing tool failure including injuries to humans. Moreover, dimensional control of the sintered part is facilitated if the pressing pressure in kept within a certain desired and practicable range.
Fatty acids and their salts and esters are long known in industry for their lubricant properties. They are sometimes characterized by the length of their carbon chains. Oleic acid and stearic acid are both 18 carbon chain equivalents often referred to as C-18 and erucic acid and behenic acid have one of the longest carbon chains in naturally occurring fatty acids (C-22).
A method of lowering the compacting pressure for submicron cemented carbide is disclosed in EP-A-1043413. The method consists in premixing all components except WC for about three hours, adding the WC powder and then finally milling for about ten hours.